This invention relates to a method and apparatus for treating hazardous wastes or other hydrocarbon-bearing materials, such as municipal wastes or coal, so that the final product is rendered innocuous or made more valuable. Hazardous wastes suitable for processing using this invention include soils and earth contaminated with fuels, solvents, etc.; sludges produced from various chemical and petrochemical processes such as desalting sludges and still bottoms; filter cakes containing organics such as those produced in the ink and dry cleaning industries; and waste solvents. Alternatively, where appropriate, valuable products can be recovered wherein the residues are easily disposable and the effluents are readily treated. In particular, the invention relates to a method and apparatus for using radio frequency energy to heat waste materials, drive off effluents including water, volatilize hydrocarbons and the like, and inactivate microorganisms and possibly pyrolyze dried hazardous waste in order to produce a harmless residue of reduced volume for disposal and an easily treatable effluent. Radio frequency heating reduces the solids volume so that relatively little space is needed for long term and safe storage of the reduced volume residue. In many cases the results of the heating generate effluents and residues which are either commercially valuable or easily treated.
Some of the conventional processes for treating hazardous waste, such as incineration or thermal treatment using steam or hot gases, produce additional waste gases which must be disposed of properly. Alternative methods such as solidification of the waste followed by disposal in landfills, are expensive and occupy considerable quantities of space in the landfill. Additionally, disposal of solidified waste in landfills may result in long-term migration of the hazardous components into groundwater sources due to deterioration of the landfill liner.
It is known that heating waste materials drives off water and reduces the volume of the waste. There have been various proposals to use various heat sources first to dehydrate the wastes and then subsequently further treat the wastes to render hazardous materials innocuous or to recover valuable products. Such heating methods include the use of electrical heaters, combustors, incinerators, and fluidized bed heat transfer methods. These techniques generally lead to the production of considerable amounts of waste gases. In many cases such waste gases may contain organic toxic compounds or radioactive components, and these are often vented into the atmosphere. As such, they may pose significant environmental and health hazards. Other types of heating methods such as the use of heater kilns, generally result in over and underheating. This lack of temperature control generates new species of products of unknown health hazards. Such new products pose major difficulties in obtaining permits for such processes. Such nonuniform heating processes are also subject to mechanical failure due to sticking or the development of scale or char in the apparatus.
It has been proposed that it may be easier to heat wastes such as radioactive contaminated ion-exchange resins uniformly by exposing them to microwave power, rather than by using conventional heat sources; however, using microwaves to treat resin presents a number of other problems. If a mass of resin is heated by microwaves using conventional apparatus, heat loss from the resin mass at its surfaces due to conduction, convection or radiation results in temperature variations throughout the resin, which is highly undesirable. In Bridges, J. E. et al., "RF/Microwave Volume-Reduction and Stabilization System For Radwaste Resins," presented at the Waste Management 1986 Symposium on Radioactive Waste Management, it was disclosed that radio frequency energy and microwave energy produced by an RF power source and fed to an RF reactor, which could be sealed with radioactively contaminated ion exchange resin inside, may be used for the volume reduction and stabilization of those resins. In particular, it was there suggested that radioactive ion exchange resin beads can be treated by exposing them to microwave radiation, which drives off water left over from ion exchange processes. Effluents from the RF reactor are fed to a vapor/liquid separator; gases from the vapor/liquid separator are received by a causticized carbon absorber; and liquids are pumped from the bottom of the separator to an activated carbon absorber.
Other methods of heating materials are known for use in different environments. For instance, see Bridges et al., U.S. Reissue Pat. No. Re. 30,738 for Apparatus and Method for In Situ Heat Processing of Hydrocarbonaceous Formations, which discloses a method and an apparatus for heating buried materials, such as oil shale, bitumen, and the like. The apparatus employs a central exciter plate or equivalent electrode and a pair of grounded plates or equivalent electrodes outside it. The electrodes receive radio frequency energy and produce a transversely uniform radio frequency electric field for heating, by displacement current, the buried material bounded by the electrodes. In order to provide even more uniform heating of the buried deposits by such an apparatus, a method of and apparatus may be used for repetitively switching or altering the electrical termination of the electrodes of the apparatus to vary the longitudinal standing wave patterns of the electric field, as disclosed in U.S. Pat. No. 4,449,585 to Bridges et al. for Apparatus and Method for In Situ Controlled Heat Processing of Hydrocarbonaceous Formations.
Considerable prior art exists relative to drying paper and wood products, sand cores for casting, drying foods, as well as setting resins in plywood, vulcanization of rubber, cooking bacon, sulfur removal from coal, or as a replacement for evaporators or thin-film dryers. In general, such applications have emphasized the use of microwave energy except in the case of drying processes for textiles, paper, wood products and the setting of molds. Such applications have not required careful control of the emitted vapors and generally rely on the vaporization of water to terminate the heating process.
Typical of microwave processes are those described in "Microwave Drying of Coal" published by David Lindroff of the Twin Cities Research Center, Bureau of Mines, Minneapolis, Minn. Comparable arrangements have been proposed by Jeppson U.S. Pat. Nos. 4,619,005 and 4,252,487 as well as Sindelar U.S. Pat. No. 4,347,016 and by Pickerman U.S. Pat. No. 4,256,093. The latter patents address the problem of heating asphalt fragments by microwave energy for recycling of asphalt paving.
Kirkbride U.S. Pat. No. 4,148,614 proposed an arrangement similar to that described by Lindroff wherein coal is exposed to microwave radiation with the objective of reducing the sulfur content. Such microwave systems appear to be impractical for large scale processing owing to the reduced depth of penetration of the microwave energy into the coal along with problems of refluxing and vapor control associated with the high temperatures needed to desulfurize the coal.
In all such above cases, no control of the vapors or effluents is taught. No features are offered to prevent heat loss to the walls of the radio frequency or microwave heating unit.
Others such as Hanson U.S. Pat. No. 4,389,076 proposed heating hydrocarbonaceous material such as oil shale by passing the oil shale through a ceramic tube which penetrates cleanly through a microwave oven or cavity. Other complex systems have been proposed by Hodge U.S. Pat. No. 2,542,028, Schlesman U.S. Pat. No. 2,486,684, and Wall U.S. Pat. No. 4,376,034. All such arrangements suffer from the aforementioned difficulties; however, Wall discloses a feature which uses a preheating step whereby the vapors derived from the hot shale are allowed to condense on the cooler incoming shale as a technique to preheat the oil shale and to conserve energy. Such a process leads to the generation of unwanted species since the same molecule is progressively reheated several times which can result in products with very high pour points or with highly toxic or carcinogenic properties. Wall also proposes to use an air-tight chamber with suitable air-locks on the incoming and output ports; however, he does not describe a system where the vapors are progressively drawn off from different temperature regions of the material, nor does he describe a lower frequency system which is more suitable for processing large volumes of material, nor does he describe the need for fluid uniformity or other means to obtain uniform temperatures.
Calderon U.S. Pat. No. 4,376,033 discloses heating oil shale by thermal conduction from the hot walls. In this instance, no high frequency or microwave heating process is employed. Calderon proposes to use electrical induction heating to heat the walls.
Wear U.S. Pat. No. 4,746,968 discloses a combination microwave drying cavity and an infrared thermal radiator that complements the microwave radiation to effect a more uniform drying of a product within the cavity. In the case of Wear, the material when heated above 100.degree. C. apparently lost nearly all of its ability to absorb microwave energy. As a consequence, the infrared heaters were used to provide additional heating. Thus, in Wear's case, energy is transferred from a heated plate by irradiation to the material. If the surface is too hot, the material becomes sticky and gummy and thereby eventually clogs the mechanics of the system. On the other hand, if the wall material is significantly lower than that of the material being processed, energy is lost from the material being processed. In the case of wet or moist material where a high energy absorption occurs, this may not be a significant problem but it can be significant in the case of very dry materials. These have little absorbing ability and therefore have little capability to simultaneously heat themselves and the adjacent walls.